1. Field of the Invention
This invention relates in general to oil and gas well equipment, and in particular to a device for setting a graphite seal between casing and a wellhead housing.
2. Description of the Prior Art
In an oil and gas well, a wellhead housing of some type will be located at the surface. Casing will extend through the wellhead housing and be cemented in place. In one type of installation, a casing hanger is secured to the upper end of the casing. The casing hanger has a smooth external surface. A seal or packoff seals the annular space between the casing hanger external surface and the wellhead.
In another type of installation, the seal locates between the wellhead housing and the casing itself. This installation may be used when the casing sticks in the well, and has to be cut off in a place such that a casing hanger cannot be used.
One type of seal for sealing against casing has a graphite central section sandwiched between upper and lower metal rings. The seal will be located on top of a shoulder in the bore of the wellhead housing. An upper wellhead housing, such as a tubing head, bolts to the lower wellhead housing. The upper wellhead housing has a downward facing shoulder that engages the seal. As the upper wellhead housing is secured in place, it will compress the seal, causing it to expand and seal against the casing.
While this is satisfactory, it does not allow the compression on the seal to be precisely set. The compression force exerted on the seal results from the difference in axial dimension between the seal initially and the shoulders in the upper and lower wellhead housings.